Further experiments will be undertaken to characterize the contractile ring, the microfilamentous organelle which constitutes the cell's device for causing cleavage constriction during cell division. Aspcts of its ultrastructure, biochemical composition, mode of action, and metabolic requirements will be examined. Particular emphasis will be placed on the detection of actomyosin-like ATPase activity by histochemical procedures, on the localization of myosin- analoques by immuno-electron microscopy, and on attempts to reactivate contraction in extracted models of cells during division. The mechanism of cleavage stimulation will be investigated by attempting to support a hypothesis of calcium-activated contraction. Localization of calcium-concentrating vesicles in dividing cells (marine eggs and cultured mammalian cells) will constitute a major project. Also, various pharmacologic agents which affect calcium metabolism will be employed as experimntal probes to the process of cleavage contraction and its stimulus.